1. Field of the Invention
The invention relates to a display unit for displaying an image on a display device, wherein a plurality of periods, called sub-fields, are defined, each sub-field having a respective illumination level which is applied to the display device.
The invention further relates to an image display apparatus comprising such a display unit.
The invention further relates to method of displaying an image on a display device, wherein a plurality of periods, called sub-fields, are defined, each sub-field having a respective illumination level which is applied to the display device.
2. Description of the Related Art
U.S. Pat. No. 5,841,413 describes a plasma display panel driven in a plurality of sub-fields. A plasma display panel is made up of a number of cells that can be switched on and switched off. A cell corresponds with a pixel (picture element) of the image that is to be displayed on the panel. In the operation of the plasma display panel, three phases can be distinguished. The first phase is the erasure phase in which the memories of all cells of the panel are erased. The second phase is the addressing phase, in which the cells of the panel that are to be switched on are conditioned by setting appropriate voltages on their electrodes. The third phase is the sustain phase, in which sustain pulses are applied to the cells which cause the addressed cells to emit light for the duration of the sustain phase. The plasma display panel only emits light during this sustain phase. The three phases together are called a sub-field period or simply a sub-field. A single image, or frame, is displayed on the panel in a number of successive sub-field periods. A cell may be switched on for one or more of the sub-field periods. The light emitted by a cell in the sub-field periods in which it was switched on, is integrated in the eye of the viewer who perceives a corresponding intensity for that cell. In a particular sub-field period, the sustain phase is maintained for a particular time resulting in a particular illumination level of the activated cells. Typically, different sub-fields have a different duration of their sustain phase. A sub-field is given a coefficient of weight to express its contribution to the light emitted by the panel during the whole frame period. An example is a plasma display panel with 6 sub-fields having coefficients of weight of 1, 2, 4, 8, 16 and 32 respectively. By selecting the appropriate sub-fields in which a cell is switched on, 64 different intensity levels can be realized in displaying an image on this panel. The plasma display panel is then driven by using binary code words of 6 bits each, whereby a code word indicates the intensity level of a pixel in binary form.
In driving a plasma display panel, the frame period, i.e. the period between two successive images, is divided into a number of sub-field periods. During each of these sub-field periods, a cell may or may not be switched on and the integration over the sub-field periods results in a perceived intensity level of the pixel corresponding with this cell. Instead of displaying a pixel of an image as a single light flash that is proportional to its intensity level, on a plasma display panel, the pixel is displayed as a series of flashes shifted in time with respect to each other. This may cause artifacts if the eyes of the viewer move. Then it appears as if the light flashes do not originate from a single position and a blurring effect occurs. Furthermore, artifacts may occur in case the images show a moving object. The movement needs to be taken into account when displaying the object in a number of sub-fields. For each next sub-field, the object must be moved a little. Motion compensation techniques are used to calculate a corrected position for the sub-pixels in the sub-fields. In some circumstances, the motion compensation techniques are not fully reliable and may produce erroneous results, e.g. in an area of the image with little detail. The erroneous results lead to motion compensation where this should not be done. This gives so-called motion artifacts which are very visible.
An artifact is most noticeable if two neighboring pixels have a small difference in intensity level while for one of the pixels the sub-field with the largest coefficient of weight is on and for the other of the pixels this sub-field is off. In case of the example of the binary code above, the code word for one pixel has the most significant bit on and the code word for the other pixel has the most significant bit off. Any error in the calculated position of a sub-field, i.e., any motion artifact involving these pixels, will then give a relatively large artifact in the displayed image. The device described in U.S. Pat. No. 5,841,413 tries to mitigate these artifacts by restricting the code words that are used. This known device employs more sub-fields than necessary for realizing the required set of intensity values. The resulting set of code words for expressing the intensity value is redundant, i.e., for a given intensity value, more than one code word is available. From this redundant set, a subset is created whereby those code words are selected that give the least differences in the most significant bit for expressing a difference between the intensity values. This subset is created by searching the original set and determining what the effect on the artifacts may be for a difference between a given code word and each of the other code words.
It is an object of the invention to provide a display unit as described in the preamble with an improved reduction of artifacts. This object is achieved, according to the invention, in that the display unit comprises:
an input for receiving respective input intensity values for sub-pixels of a particular pixel of the image,
a control unit for:
comparing at least one of the input intensity values with at least one predetermined value,
conditionally modifying, on the basis of said comparing, the at least one of the input intensity values to a desired value, and
modifying at least a further one of the input intensity values to compensate the effect on a property of the pixel caused by the modifying, if any, of the at least one of the input intensity values,
an output for sending respective output intensity values on the basis of the respective input intensity values potentially modified by the control unit, and
a coding unit for coding the output intensity levels into combinations of sub-fields for the respective sub-pixels.
The display unit of the invention makes it possible to control the intensity value of a sub-pixel, i.e., to modify it from its original intensity value to a desired value, while the effect that such modification would have on a given property of the pixel of which this sub-pixel forms a part, is compensated by a change of the intensity value for one of the other sub-pixels of that pixel. According to the invention, a flexibility is created to change the intensity value of one of the sub-pixels while this property does not change, by also changing the intensity value of one of the other colors. This property can be the luminance of a pixel, the color of a pixel, or some other characteristic of the pixel realized by the contribution of the sub-pixels of the pixel.
Controlling the intensity value that is sent to the display device for a certain sub-pixel, gives direct control whether a specific sub-field for that sub-pixel is switched on or not. This makes it possible to avoid the above problems where two nearby pixels have almost the same intensity value while one has a high weighted sub-field on while the other has not. The intensity value for one of the pixels is controlled in such a way that both have the high weighted sub-field on or off, whichever is most suitable in the situation at hand. The display unit of the invention has the advantage that it can be applied to a scheme of sub-field weights where the number of possible intensity level is maximal in view of the number of sub-fields, while in the known device the number of sub-fields has been increased for a given number of intensity levels. An example of such a favorable scheme is the binary distribution, where the sub-fields weights are powers of 2.
In an embodiment of the display unit according to the invention as described above, the property of the pixel is the luminance of the pixel. The display unit of this embodiment makes it possible to control the intensity value of a color sub-pixel, i.e., to modify it from its original intensity value to a desired value, while the effect that such modification would have on the luminance of the pixel of which this color sub-pixel forms a part, is compensated by a change of the intensity value for one of the other sub-pixels of that pixel. According to the invention, a flexibility is created to change the intensity value of one of the colors while the luminance does not change, by also changing the intensity value of one of the other colors. This introduces a color error, but the human visual system is less sensitive to color changes than to luminance changes. It is reported that the smallest change in luminance a human can perceive is a change of 2%, while this is 5% for a change in color.
In an embodiment of the display unit according to the invention as described above, the input is arranged to receive the input intensity values for red, green and blue respectively and wherein the control unit is arranged to conditionally modify at least one of the 3 input intensity values to control its value and to modify at least one of the other 2 input intensity values to compensate the effect on the luminance according to the following equation:
0.3xcex94R+0.59xcex94G+0.11xcex94B=0
wherein xcex94R is the modification of the red intensity value, xcex94G is the modification of the green intensity value, and xcex94B is the modification of the blue intensity value. The control unit of this embodiment compensates the effect on the luminance of the modification of the first color with a modification of a further color by applying a simple relation expressing the respective contributions of the colors to the perceived luminance.
In an embodiment of the display unit according to the invention as described above, the predetermined value corresponds with the illumination level of the highest weighted sub-field. This makes it possible to control the activation of the highest weighted sub-field of the corresponding color sub-pixel.
In an embodiment of the display unit according to the invention as described above, the control unit is arranged to modify the at least one of the input intensity values if it falls in a range with a lower boundary equal to the predetermined value minus xcex94in and an upper boundary equal to the predetermined value plus xcex94in, xcex94in being equal to 5% of the maximum intensity level. By limiting the modification of the at least one of the input intensity values to this range, the compensating modification of the further one of the input intensity values is kept relatively small. This limits the color change of the pixel to a level that, in many practical situations, cannot be perceived by the human visual system.
In an embodiment of the display unit according to the invention as described above, the control unit is arranged to compare the modification of the further one of the input intensity values with a limit and, if it exceeds the limit, to disregard the modifications and to output the input intensity values as the output intensity values. The control unit in this embodiment avoids the generation of output intensity values of which the resulting color is so different from the original input intensity values, that it can easily be perceived. Rather than generating those output intensity values, the control unit outputs the original input intensity values. The control over the intensity value of the color sub-pixel is not carried out since it would result in an image that is perceivably worse than the original image.
It is a further object of the invention to provide method as described in the preamble realizing an improved reduction of artifacts. This object is achieved according to the invention in that the method comprises:
an input step of receiving respective input intensity values for sub-pixels of a particular pixel of the image,
a control step comprising:
comparing at least one of the input intensity values with a predetermined value,
conditionally modifying on the basis of said comparing the at least one of the input intensity values to a desired value, and
modifying at least a further one of the input intensity values to compensate the effect on a property of the pixel caused by the modifying, if any, of the at least one of the input intensity values,
an output step of sending respective output intensity values on the basis of the respective input intensity values potentially modified by the control unit, and
a coding step of coding the output intensity levels into combinations of sub-fields for the respective sub-pixels.